一、背景

使用Spring cloud开发微服务系统，一般采用的架构如下所示，Gateway不仅作为请求转发的入口，还会进行权限验证、监控、缓存、请求路由、流量限制、请求熔断、请求重试等功能。Spring Cloud Gateway作为Spring Cloud体系中API网关。

1. 核心概念

Spring Cloud GateWay 最主要的功能就是路由转发，而在定义转发规则时主要涉及了以下三个核心概念：

• Route
  网关最基本的模块。它由一个 ID、一个目标 URI、一组断言（Predicate）和一组过滤器（Filter）组成。
• Predicate
  路由转发的判断条件，我们可以通过 Predicate 对 HTTP 请求进行匹配，例如请求方式、请求路径、请求头、参数等，如果请求与断言匹配成功，则将请求转发到相应的服务。
  Route 路由与 Predicate 断言的对应关系为“一对多”，一个路由可以包含多个不同断言。一个请求想要转发到指定的路由上，就必须同时匹配路由上的所有断言。当一个请求同时满足多个路由的断言条件时，请求只会被首个成功匹配的路由转发。
• Filter
  过滤器，我们可以使用它对请求进行拦截和修改，还可以使用它对上文的响应进行再处理。

2. 工作流程

Spring Cloud Gateway 工作流程说明如下：

• 客户端将请求发送到 Spring Cloud Gateway 上。
• Spring Cloud Gateway 通过 Gateway Handler Mapping 找到与请求相匹配的路由，将其发送给 Gateway Web Handler。
• Gateway Web Handler 通过指定的过滤器链（Filter Chain），将请求转发到实际的服务节点中，执行业务逻辑返回响应结果。
• 过滤器之间用虚线分开是因为过滤器可能会在转发请求之前（pre）或之后（post）执行业务逻辑。
• 过滤器（Filter）可以在请求被转发到服务端前，对请求进行拦截和修改，例如参数校验、权限校验、流量监控、日志输出以及协议转换等。
• 过滤器可以在响应返回客户端之前，对响应进行拦截和再处理，例如修改响应内容或响应头、日志输出、流量监控等。
  响应原路返回给客户端。

3.动态路由

默认情况下，Spring Cloud Gateway 会根据服务注册中心（例如 Eureka Server）中维护的服务列表，以服务名（spring.application.name）作为路径创建动态路由进行转发，从而实现动态路由功能。可以在配置文件中，将 Route 的 uri 地址修改为以下形式。

lb://service-name

以上配置说明如下：
lb：uri 的协议，表示开启 Spring Cloud Gateway 的负载均衡功能。
service-name：服务名，Spring Cloud Gateway 会根据它获取到具体的微服务地址。

二、自动配置源码解析

1.GatewayAutoConfiguration

@Configuration(proxyBeanMethods = false)
@ConditionalOnProperty(name = "spring.cloud.gateway.enabled", matchIfMissing = true)
@EnableConfigurationProperties
// HttpHandlerAutoConfiguration.class 中自动配置了HttpHandler
// WebFluxAutoConfiguration.class 中自动配置了DispatcherHandler
@AutoConfigureBefore({ HttpHandlerAutoConfiguration.class, WebFluxAutoConfiguration.class })

@AutoConfigureAfter({ GatewayReactiveLoadBalancerClientAutoConfiguration.class,
		GatewayClassPathWarningAutoConfiguration.class })
@ConditionalOnClass(DispatcherHandler.class)
public class GatewayAutoConfiguration {
	
	@Bean
	@Primary
	@ConditionalOnMissingBean(name = "cachedCompositeRouteLocator")
	// TODO: property to disable composite?
	public RouteLocator cachedCompositeRouteLocator(List<RouteLocator> routeLocators) {
		// yml文件routes配置内容
		return new CachingRouteLocator(new CompositeRouteLocator(Flux.fromIterable(routeLocators)));
	}

	@Bean
	public FilteringWebHandler filteringWebHandler(List<GlobalFilter> globalFilters) {
		// 创建FilteringWebHandler, 即RoutePredicateHandlerMapping#webHandler
		return new FilteringWebHandler(globalFilters);
	}

	@Bean
	@ConditionalOnMissingBean
	public RoutePredicateHandlerMapping routePredicateHandlerMapping(FilteringWebHandler webHandler,
			RouteLocator routeLocator, GlobalCorsProperties globalCorsProperties, Environment environment) {
		// DispatcherHandler中HandlerMapping 之一，实际进行HandlerMapping匹配
		return new RoutePredicateHandlerMapping(webHandler, routeLocator, globalCorsProperties, environment);
	}

	...

}

2.WebFluxAutoConfiguration

@AutoConfiguration(after = { ReactiveWebServerFactoryAutoConfiguration.class, CodecsAutoConfiguration.class,
		ReactiveMultipartAutoConfiguration.class, ValidationAutoConfiguration.class,
		WebSessionIdResolverAutoConfiguration.class })
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
@ConditionalOnClass(WebFluxConfigurer.class)
@ConditionalOnMissingBean({ WebFluxConfigurationSupport.class })
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
public class WebFluxAutoConfiguration {
	... 
	
	/**
	 * Configuration equivalent to {@code @EnableWebFlux}.
	 */
	@Configuration(proxyBeanMethods = false)
	@EnableConfigurationProperties({ WebProperties.class, ServerProperties.class })
	public static class EnableWebFluxConfiguration extends DelegatingWebFluxConfiguration {
}
}

EnableWebFluxConfiguration 的继承关系如下：

DispatcherHandler在WebFluxConfigurationSupport中进行实例化

	@Bean
	public DispatcherHandler webHandler() {
		// 实际将http请求，装换到HttpWebHandlerAdapter进行处理
		return new DispatcherHandler();
	}

3.HttpHandlerAutoConfiguration

@AutoConfiguration(after = { WebFluxAutoConfiguration.class })
@ConditionalOnClass({ DispatcherHandler.class, HttpHandler.class })
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.REACTIVE)
@ConditionalOnMissingBean(HttpHandler.class)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE + 10)
public class HttpHandlerAutoConfiguration {

	@Configuration(proxyBeanMethods = false)
	public static class AnnotationConfig {

		private final ApplicationContext applicationContext;

		public AnnotationConfig(ApplicationContext applicationContext) {
			this.applicationContext = applicationContext;
		}

		@Bean
		public HttpHandler httpHandler(ObjectProvider<WebFluxProperties> propsProvider) {
			// 创建HttpHanlder
			HttpHandler httpHandler = WebHttpHandlerBuilder.applicationContext(this.applicationContext).build();
			WebFluxProperties properties = propsProvider.getIfAvailable();
			if (properties != null && StringUtils.hasText(properties.getBasePath())) {
				Map<String, HttpHandler> handlersMap = Collections.singletonMap(properties.getBasePath(), httpHandler);
				return new ContextPathCompositeHandler(handlersMap);
			}
			return httpHandler;
		}

	}

}

public static WebHttpHandlerBuilder applicationContext(ApplicationContext context) {
		// 获取名称为WEB_HANDLER_BEAN_NAME，类型为WebHandler的Bean
		WebHttpHandlerBuilder builder = new WebHttpHandlerBuilder(
				context.getBean(WEB_HANDLER_BEAN_NAME, WebHandler.class), context);
		
		// 获取WebFilter类型的Filter
		List<WebFilter> webFilters = context
				.getBeanProvider(WebFilter.class)
				.orderedStream()
				.collect(Collectors.toList());
		builder.filters(filters -> filters.addAll(webFilters));
		// 获取WebExceptionHandler类型的Handler
		List<WebExceptionHandler> exceptionHandlers = context
				.getBeanProvider(WebExceptionHandler.class)
				.orderedStream()
				.collect(Collectors.toList());
		builder.exceptionHandlers(handlers -> handlers.addAll(exceptionHandlers));

		context.getBeanProvider(HttpHandlerDecoratorFactory.class)
				.orderedStream()
				.forEach(builder::httpHandlerDecorator);

		try {
			// 设置session管理器
			builder.sessionManager(
					context.getBean(WEB_SESSION_MANAGER_BEAN_NAME, WebSessionManager.class));
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Fall back on default
		}

		try {
			// 设置编解码器
			builder.codecConfigurer(
					context.getBean(SERVER_CODEC_CONFIGURER_BEAN_NAME, ServerCodecConfigurer.class));
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Fall back on default
		}

		try {
			builder.localeContextResolver(
					context.getBean(LOCALE_CONTEXT_RESOLVER_BEAN_NAME, LocaleContextResolver.class));
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Fall back on default
		}

		try {
			builder.forwardedHeaderTransformer(
					context.getBean(FORWARDED_HEADER_TRANSFORMER_BEAN_NAME, ForwardedHeaderTransformer.class));
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Fall back on default
		}

		return builder;
	}

	// 生成HttpBuilder
	public HttpHandler build() {
		// 将DispatcherHandler，和过滤器组合使用装饰器组成WebHandler
		WebHandler decorated = new FilteringWebHandler(this.webHandler, this.filters);
		// 再将exceptionHandlers放到装饰器，组成ExceptionHandlingWebHandler
		decorated = new ExceptionHandlingWebHandler(decorated,  this.exceptionHandlers);
		// 将WebHandler通过转换器实现HttpHandler的handle方法
		HttpWebHandlerAdapter adapted = new HttpWebHandlerAdapter(decorated);
		if (this.sessionManager != null) {
			// 设置session管理器
			adapted.setSessionManager(this.sessionManager);
		}
		if (this.codecConfigurer != null) {
			// 设置编解码器
			adapted.setCodecConfigurer(this.codecConfigurer);
		}
		if (this.localeContextResolver != null) {
			// 设置LocaleContextResolver
			adapted.setLocaleContextResolver(this.localeContextResolver);
		}
		if (this.forwardedHeaderTransformer != null) {
			// 设置http hedaer 转换器
			adapted.setForwardedHeaderTransformer(this.forwardedHeaderTransformer);
		}
		if (this.applicationContext != null) {
			// 设置applicationContext
			adapted.setApplicationContext(this.applicationContext);
		}
		adapted.afterPropertiesSet();

		return (this.httpHandlerDecorator != null ? this.httpHandlerDecorator.apply(adapted) : adapted);
	}

三、请求过程源码解析

1. 整体流程

下图的请求过程，是对Spring Cloud Gateway工作流程的细化

2. NettyWebServer接收http请求

NettyWebServer是通过HttpServer来接收http请求

public void onStateChange(Connection connection, State newState) {
			// 接收到http 请求事件，则进行处理
			if (newState == HttpServerState.REQUEST_RECEIVED) {
				try {
					if (log.isDebugEnabled()) {
						log.debug(format(connection.channel(), "Handler is being applied: {}"), handler);
					}
					HttpServerOperations ops = (HttpServerOperations) connection;
					// 调用ReactorHttpHandlerAdapter进行请求处理
					Publisher<Void> publisher = handler.apply(ops, ops);
					Mono<Void> mono = Mono.deferContextual(ctx -> {
						ops.currentContext = Context.of(ctx);
						return Mono.fromDirect(publisher);
					});
					if (ops.mapHandle != null) {
						mono = ops.mapHandle.apply(mono, connection);
					}
					mono.subscribe(ops.disposeSubscriber());
				}
				catch (Throwable t) {
					log.error(format(connection.channel(), ""), t);
					//"FutureReturnValueIgnored" this is deliberate
					connection.channel()
					          .close();
				}
			}
		}

3.请求和响应转换

ReactorHttpHandlerAdapter主要是将NettyServer中http的请求和响应，装换为ReactorServerHttpRequest 和 ReactorServerHttpResponse。

@Override
	public Mono<Void> apply(HttpServerRequest reactorRequest, HttpServerResponse reactorResponse) {
		NettyDataBufferFactory bufferFactory = new NettyDataBufferFactory(reactorResponse.alloc());
		try {
			// 将NettyServer中http的请求和响应，装换为ReactorServerHttpRequest 和 ReactorServerHttpResponse
			ReactorServerHttpRequest request = new ReactorServerHttpRequest(reactorRequest, bufferFactory);
			ServerHttpResponse response = new ReactorServerHttpResponse(reactorResponse, bufferFactory);

			if (request.getMethod() == HttpMethod.HEAD) {
				response = new HttpHeadResponseDecorator(response);
			}
			// 调用DelayedInitializationHttpHandler进行http请求处理，DelayedInitializationHttpHandler直接将请求转给HttpWebHandlerAdapter
			return this.httpHandler.handle(request, response)
					.doOnError(ex -> logger.trace(request.getLogPrefix() + "Failed to complete: " + ex.getMessage()))
					.doOnSuccess(aVoid -> logger.trace(request.getLogPrefix() + "Handling completed"));
		}
		catch (URISyntaxException ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Failed to get request URI: " + ex.getMessage());
			}
			reactorResponse.status(HttpResponseStatus.BAD_REQUEST);
			return Mono.empty();
		}
	}

4.HttpWebHandlerAdapter

@Override
	public Mono<Void> handle(ServerHttpRequest request, ServerHttpResponse response) {
		if (this.forwardedHeaderTransformer != null) {
			try {
				// 处理http 请求头中：Forwarded、X-Forwarded-Host、X-Forwarded-Port、X-Forwarded-Proto、X-Forwarded-Prefix、X-Forwarded-Ssl、X-Forwarded-For，包装请求来覆盖其host、端口和scheme方案
				request = this.forwardedHeaderTransformer.apply(request);
			}
			catch (Throwable ex) {
				if (logger.isDebugEnabled()) {
					logger.debug("Failed to apply forwarded headers to " + formatRequest(request), ex);
				}
				response.setStatusCode(HttpStatus.BAD_REQUEST);
				return response.setComplete();
			}
		}
		// 将http 请求和响应，装换为DefaultServerWebExchange
		ServerWebExchange exchange = createExchange(request, response);

		LogFormatUtils.traceDebug(logger, traceOn ->
				exchange.getLogPrefix() + formatRequest(exchange.getRequest()) +
						(traceOn ? ", headers=" + formatHeaders(exchange.getRequest().getHeaders()) : ""));
		// 调用ExceptionHandlingWebHandler 进行处理
		return getDelegate().handle(exchange)
				.doOnSuccess(aVoid -> logResponse(exchange))
				.onErrorResume(ex -> handleUnresolvedError(exchange, ex))
				.then(Mono.defer(response::setComplete));
	}

5. ExceptionHandlingWebHandler

	@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		Mono<Void> completion;
		try {
			// 调用DispatcherHandler进行处理
			completion = super.handle(exchange);
		}
		catch (Throwable ex) {
			completion = Mono.error(ex);
		}
		// 处理异常
		for (WebExceptionHandler handler : this.exceptionHandlers) {
			completion = completion.onErrorResume(ex -> handler.handle(exchange, ex));
		}
		return completion;
	}

6. DispatcherHandler

DispatcherHandler是一个很重要的处理类，处理的主流程是handle，选匹配handlerMapping，再执行invokeHandler，最后执行handleResult。

	@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		if (this.handlerMappings == null) {
			return createNotFoundError();
		}
		// Cors预请求检测：Method为Options,请求头包括：Origin、Access-Control-Request-Method，Cors详情见：https://developer.mozilla.org/zh-CN/docs/Web/HTTP/CORS
		// Cors: 浏览器必须首先使用 OPTIONS 方法发起一个预检请求（preflight request），从而获知服务端是否允许该跨源请求
		if (CorsUtils.isPreFlightRequest(exchange.getRequest())) {
			return handlePreFlight(exchange);
		}
		return Flux.fromIterable(this.handlerMappings)
				// rote通过predicates匹配，是否支持该请求
				.concatMap(mapping -> mapping.getHandler(exchange))
				.next()
				// 没有找到请求处理器，返回404
				.switchIfEmpty(createNotFoundError())
				// 调用请求
				.flatMap(handler -> invokeHandler(exchange, handler))
				// 处理返回结果
				.flatMap(result -> handleResult(exchange, result));
	}

	@Override
	public Mono<Void> handlePreFlight(ServerWebExchange exchange) {
		return Flux.fromIterable(this.handlerMappings != null ? this.handlerMappings : Collections.emptyList())
				.concatMap(mapping -> mapping.getHandler(exchange))
				// cors预请求在匹配为空，返回403
				.switchIfEmpty(Mono.fromRunnable(() -> exchange.getResponse().setStatusCode(HttpStatus.FORBIDDEN)))
				.next()
				.then();
	}
	
	private Mono<HandlerResult> invokeHandler(ServerWebExchange exchange, Object handler) {
		if (ObjectUtils.nullSafeEquals(exchange.getResponse().getStatusCode(), HttpStatus.FORBIDDEN)) {
			return Mono.empty();  // CORS rejection
		}
		if (this.handlerAdapters != null) {
			for (HandlerAdapter handlerAdapter : this.handlerAdapters) {
				// 判断handlerAdapter 是否支持 handler
				if (handlerAdapter.supports(handler)) {
					// 调用 SimpleHandlerAdapter 执行请求，实际代理为FilteringWebHandler进行执行
					return handlerAdapter.handle(exchange, handler);
				}
			}
		}
		return Mono.error(new IllegalStateException("No HandlerAdapter: " + handler));
	}

	private Mono<Void> handleResult(ServerWebExchange exchange, HandlerResult result) {
		// http响应处理
		return getResultHandler(result).handleResult(exchange, result)
				.checkpoint("Handler " + result.getHandler() + " [DispatcherHandler]")
				.onErrorResume(ex ->
						result.applyExceptionHandler(ex).flatMap(exResult -> {
							String text = "Exception handler " + exResult.getHandler() +
									", error=\"" + ex.getMessage() + "\" [DispatcherHandler]";
							return getResultHandler(exResult).handleResult(exchange, exResult).checkpoint(text);
						}));
	}

	private HandlerResultHandler getResultHandler(HandlerResult handlerResult) {
		if (this.resultHandlers != null) {
			for (HandlerResultHandler resultHandler : this.resultHandlers) {
				if (resultHandler.supports(handlerResult)) {
					return resultHandler;
				}
			}
		}
		throw new IllegalStateException("No HandlerResultHandler for " + handlerResult.getReturnValue());
	}

7.FilteringWebHandler

@Override
	public Mono<Void> handle(ServerWebExchange exchange) {
		Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
		// 获取匹配route中配置的filters
		List<GatewayFilter> gatewayFilters = route.getFilters();
		
		// 全局Filters
		List<GatewayFilter> combined = new ArrayList<>(this.globalFilters);
		// 添加route中的filters
		combined.addAll(gatewayFilters);
		// 排序
		AnnotationAwareOrderComparator.sort(combined);

		if (logger.isDebugEnabled()) {
			logger.debug("Sorted gatewayFilterFactories: " + combined);
		}
		// 创建Filter调用链
		return new DefaultGatewayFilterChain(combined).filter(exchange);
	}

	private static class DefaultGatewayFilterChain implements GatewayFilterChain {

		private final int index;

		private final List<GatewayFilter> filters;

		DefaultGatewayFilterChain(List<GatewayFilter> filters) {
			this.filters = filters;
			this.index = 0;
		}

		private DefaultGatewayFilterChain(DefaultGatewayFilterChain parent, int index) {
			this.filters = parent.getFilters();
			this.index = index;
		}

		public List<GatewayFilter> getFilters() {
			return filters;
		}

		@Override
		public Mono<Void> filter(ServerWebExchange exchange) {
			return Mono.defer(() -> {
				if (this.index < filters.size()) {
					GatewayFilter filter = filters.get(this.index);
					DefaultGatewayFilterChain chain = new DefaultGatewayFilterChain(this, this.index + 1);
					// 依次调用filter，其中最重要的是ReactiveLoadBalancerClientFilter，NettyRoutingFilter 
					return filter.filter(exchange, chain);
				}
				else {
					return Mono.empty(); // complete
				}
			});
		}

	}

8.ReactiveLoadBalancerClientFilter

这个是负载均衡Filter。

	@Override
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		URI url = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
		String schemePrefix = exchange.getAttribute(GATEWAY_SCHEME_PREFIX_ATTR);
		if (url == null || (!"lb".equals(url.getScheme()) && !"lb".equals(schemePrefix))) {
			return chain.filter(exchange);
		}
		// preserve the original url
		addOriginalRequestUrl(exchange, url);

		if (log.isTraceEnabled()) {
			log.trace(ReactiveLoadBalancerClientFilter.class.getSimpleName() + " url before: " + url);
		}

		URI requestUri = exchange.getAttribute(GATEWAY_REQUEST_URL_ATTR);
		String serviceId = requestUri.getHost();
		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = LoadBalancerLifecycleValidator
				.getSupportedLifecycleProcessors(clientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),
						RequestDataContext.class, ResponseData.class, ServiceInstance.class);
		DefaultRequest<RequestDataContext> lbRequest = new DefaultRequest<>(
				new RequestDataContext(new RequestData(exchange.getRequest()), getHint(serviceId)));
		LoadBalancerProperties loadBalancerProperties = clientFactory.getProperties(serviceId);
		// 负载均衡选择转发实例
		return choose(lbRequest, serviceId, supportedLifecycleProcessors).doOnNext(response -> {

			if (!response.hasServer()) {
				supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
						.onComplete(new CompletionContext<>(CompletionContext.Status.DISCARD, lbRequest, response)));
				throw NotFoundException.create(properties.isUse404(), "Unable to find instance for " + url.getHost());
			}

			ServiceInstance retrievedInstance = response.getServer();
			
			URI uri = exchange.getRequest().getURI();

			// if the `lb:<scheme>` mechanism was used, use `<scheme>` as the default,
			// if the loadbalancer doesn't provide one.
			String overrideScheme = retrievedInstance.isSecure() ? "https" : "http";
			if (schemePrefix != null) {
				overrideScheme = url.getScheme();
			}

			DelegatingServiceInstance serviceInstance = new DelegatingServiceInstance(retrievedInstance,
					overrideScheme);
			// 转发uri
			URI requestUrl = reconstructURI(serviceInstance, uri);

			if (log.isTraceEnabled()) {
				log.trace("LoadBalancerClientFilter url chosen: " + requestUrl);
			}
			// 设置实际转发uri
			exchange.getAttributes().put(GATEWAY_REQUEST_URL_ATTR, requestUrl);
			exchange.getAttributes().put(GATEWAY_LOADBALANCER_RESPONSE_ATTR, response);
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStartRequest(lbRequest, response));
		}).then(chain.filter(exchange))
				.doOnError(throwable -> supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
						.onComplete(new CompletionContext<ResponseData, ServiceInstance, RequestDataContext>(
								CompletionContext.Status.FAILED, throwable, lbRequest,
								exchange.getAttribute(GATEWAY_LOADBALANCER_RESPONSE_ATTR)))))
				.doOnSuccess(aVoid -> supportedLifecycleProcessors.forEach(lifecycle -> lifecycle
						.onComplete(new CompletionContext<ResponseData, ServiceInstance, RequestDataContext>(
								CompletionContext.Status.SUCCESS, lbRequest,
								exchange.getAttribute(GATEWAY_LOADBALANCER_RESPONSE_ATTR), buildResponseData(exchange,
										loadBalancerProperties.isUseRawStatusCodeInResponseData())))));
	}

9.NettyRoutingFilter

这个是实际请求http转发请求的Filter。

	@Override
	@SuppressWarnings("Duplicates")
	public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
		URI requestUrl = exchange.getRequiredAttribute(GATEWAY_REQUEST_URL_ATTR);

		String scheme = requestUrl.getScheme();
		if (isAlreadyRouted(exchange) || (!"http".equalsIgnoreCase(scheme) && !"https".equalsIgnoreCase(scheme))) {
			return chain.filter(exchange);
		}
		setAlreadyRouted(exchange);

		ServerHttpRequest request = exchange.getRequest();

		final HttpMethod method = HttpMethod.valueOf(request.getMethodValue());
		final String url = requestUrl.toASCIIString();

		HttpHeaders filtered = filterRequest(getHeadersFilters(), exchange);

		final DefaultHttpHeaders httpHeaders = new DefaultHttpHeaders();
		filtered.forEach(httpHeaders::set);

		boolean preserveHost = exchange.getAttributeOrDefault(PRESERVE_HOST_HEADER_ATTRIBUTE, false);
		Route route = exchange.getAttribute(GATEWAY_ROUTE_ATTR);

		Flux<HttpClientResponse> responseFlux = getHttpClient(route, exchange).headers(headers -> {
			headers.add(httpHeaders);
			// Will either be set below, or later by Netty
			headers.remove(HttpHeaders.HOST);
			if (preserveHost) {
				String host = request.getHeaders().getFirst(HttpHeaders.HOST);
				headers.add(HttpHeaders.HOST, host);
			}
		})
		// http请求转发
		.request(method).uri(url).send((req, nettyOutbound) -> {
			if (log.isTraceEnabled()) {
				nettyOutbound.withConnection(connection -> log.trace("outbound route: "
						+ connection.channel().id().asShortText() + ", inbound: " + exchange.getLogPrefix()));
			}
			return nettyOutbound.send(request.getBody().map(this::getByteBuf));
		}).responseConnection((res, connection) -> {

			// Defer committing the response until all route filters have run
			// Put client response as ServerWebExchange attribute and write
			// response later NettyWriteResponseFilter
			exchange.getAttributes().put(CLIENT_RESPONSE_ATTR, res);
			exchange.getAttributes().put(CLIENT_RESPONSE_CONN_ATTR, connection);

			ServerHttpResponse response = exchange.getResponse();
			// put headers and status so filters can modify the response
			HttpHeaders headers = new HttpHeaders();

			res.responseHeaders().forEach(entry -> headers.add(entry.getKey(), entry.getValue()));

			String contentTypeValue = headers.getFirst(HttpHeaders.CONTENT_TYPE);
			if (StringUtils.hasLength(contentTypeValue)) {
				exchange.getAttributes().put(ORIGINAL_RESPONSE_CONTENT_TYPE_ATTR, contentTypeValue);
			}

			setResponseStatus(res, response);

			// make sure headers filters run after setting status so it is
			// available in response
			HttpHeaders filteredResponseHeaders = HttpHeadersFilter.filter(getHeadersFilters(), headers, exchange,
					Type.RESPONSE);

			if (!filteredResponseHeaders.containsKey(HttpHeaders.TRANSFER_ENCODING)
					&& filteredResponseHeaders.containsKey(HttpHeaders.CONTENT_LENGTH)) {
				// It is not valid to have both the transfer-encoding header and
				// the content-length header.
				// Remove the transfer-encoding header in the response if the
				// content-length header is present.
				response.getHeaders().remove(HttpHeaders.TRANSFER_ENCODING);
			}

			exchange.getAttributes().put(CLIENT_RESPONSE_HEADER_NAMES, filteredResponseHeaders.keySet());

			response.getHeaders().addAll(filteredResponseHeaders);

			return Mono.just(res);
		});

		Duration responseTimeout = getResponseTimeout(route);
		if (responseTimeout != null) {
			responseFlux = responseFlux
					.timeout(responseTimeout,
							Mono.error(new TimeoutException("Response took longer than timeout: " + responseTimeout)))
					.onErrorMap(TimeoutException.class,
							th -> new ResponseStatusException(HttpStatus.GATEWAY_TIMEOUT, th.getMessage(), th));
		}

		return responseFlux.then(chain.filter(exchange));
	}